Substrate dividing method

ABSTRACT

A substrate dividing method which can thin and divide a substrate while preventing chipping and cracking from occurring. This substrate dividing method comprises the steps of irradiating a semiconductor substrate  1  having a front face  3  formed with functional devices  19  with laser light while positioning a light-converging point within the substrate, so as to form a modified region including a molten processed region due to multiphoton absorption within the semiconductor substrate  1 , and causing the modified region including the molten processed region to form a starting point region for cutting; and grinding a rear face  21  of the semiconductor substrate  1  after the step of forming the starting point region for cutting such that the semiconductor substrate  1  attains a predetermined thickness.

This is a continuation application of copending application Ser. No.10/507,321, filed on Sep. 10, 2004, which is the National Stage ofInternational Application No. PCT/JP03/02669 filed Mar. 6, 2003,designating the U.S.A., the entire contents of each of which areincorporated herein in their entirety.

TECHNICAL FIELD

The present invention relates to a substrate dividing method used fordividing a substrate such as a semiconductor substrate in a step ofmaking a semiconductor device or the like.

BACKGROUND ART

As semiconductor devices have been becoming smaller in recent years,there are cases where a semiconductor substrate is thinned to athickness of several tens of micrometers in a step of making asemiconductor device. When thus thinned semiconductor substrate is cutand divided by a blade, chipping and cracking occur more than in thecase where a semiconductor substrate is thicker, thereby causing aproblem that the yield of semiconductor chips obtained by dividing thesemiconductor substrate decreases.

Known as semiconductor substrate dividing methods which can solve such aproblem are those described in Japanese Patent Application Laid-OpenNos. SHO 64-38209 and SHO 62-4341.

In the methods described in these publications, a semiconductorsubstrate having a front face formed with a functional device isinscribed with a groove by a blade on the front face side, then anadhesive sheet is attached to the front face, so as to hold thesemiconductor substrate, and the rear face of the semiconductorsubstrate is ground until the groove formed beforehand is exposed,thereby thinning the semiconductor substrate and dividing thesemiconductor substrate.

DISCLOSURE OF THE INVENTION

If the grinding of the rear face of the semiconductor substrate isperformed by surface grinding in the methods described in theabove-mentioned publications, however, chipping and cracking may occurat side faces of the groove formed beforehand in the semiconductorsubstrate when the surface-ground face reaches the groove.

In view of such a circumstance, it is an object of the present inventionto provide a substrate dividing method which can prevent chipping andcracking from occurring, and thin and divide a substrate.

For achieving the above-mentioned object, the substrate dividing methodin accordance with the present invention comprises the steps ofirradiating a substrate with laser light while positioning alight-converging point within the substrate, so as to form a modifiedregion due to multiphoton absorption within the substrate, and causingthe modified region to form a starting point region for cutting along aline along which the substrate should be cut in the substrate inside bya predetermined distance from a laser light incident face of thesubstrate; and grinding the substrate after the step of forming thestarting point region for cutting such that the substrate attains apredetermined thickness.

Since this substrate dividing method irradiates the substrate with laserlight while positioning a light-converging point within the substrate inthe step of forming a starting point region for cutting, so as togenerate a phenomenon of multiphoton absorption within the substrate,thereby forming a modified region, this modified region can form astarting point region for cutting within the substrate along a desirableline along which the substrate should be cut for cutting the substrate.When a starting point region for cutting is formed within the substrate,a fracture is generated in the substrate in its thickness direction fromthe starting point region for cutting acting as a start point naturallyor with a relatively small force exerted thereon.

In the step of grinding the substrate, the substrate is ground such thatthe substrate attains a predetermined thickness after the starting pointregion for cutting is formed within the substrate. Here, even when theground surface reaches the fracture generated from the starting pointregion for cutting acting as a start point, cut surfaces of thesubstrate cut by the fracture remain in close contact with each other,whereby the substrate can be prevented from chipping and cracking upongrinding.

This can prevent chipping and cracking from occurring, and can thin anddivide the substrate.

Here, the light-converging point refers to a location at which laserlight is converged. The grinding encompasses shaving, polishing,chemical etching, and the like. The starting point region for cuttingrefers to a region to become a start point for cutting when thesubstrate is cut. Therefore, the starting point region for cutting is apart to cut where cutting is to be performed in the substrate. Thestarting point region for cutting may be produced by continuouslyforming a modified region or intermittently forming a modified region.

The substrate encompasses semiconductor substrates such as siliconsubstrates and GaAs substrates, and insulating substrates such assapphire substrates and AlN substrates. When the substrate is asemiconductor substrate, an example of the modified region is a moltenprocessed region.

Preferably, a front face of the substrate is formed with a functionaldevice, and a rear face of the substrate is ground in the step ofgrinding the substrate. Since the substrate can be ground after formingthe functional device, a chip thinned so as to conform to a smaller sizeof a semiconductor device, for example, can be obtained. Here, thefunctional device refers to light-receiving devices such as photodiodes,light-emitting devices such as laser diodes, circuit devices formed ascircuits, etc.

Preferably, the step of grinding the substrate includes a step ofsubjecting the rear face of the substrate to chemical etching. When therear face of the substrate is subjected to chemical etching, the rearface of the substrate becomes smoother as a matter of course. Also,since the cut surfaces of the substrate cut by the fracture generatedfrom the starting point region for cutting acting as a start pointremain in close contact with each other, only edge parts on the rearface of the cut surfaces are selectively etched, so as to be chamfered.This can improve the transverse rupture strength of chips obtained bydividing the substrate, and prevent chipping and cracking from occurringin the chips.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an object to be processed during laserprocessing in the laser processing method in accordance with anembodiment of the present invention;

FIG. 2 is a sectional view of the object to be processed taken along theline II-II of FIG. 1;

FIG. 3 is a plan view of the object to be processed after laserprocessing by the laser processing method in accordance with theembodiment;

FIG. 4 is a sectional view of the object to be processed taken along theline IV-IV of FIG. 3;

FIG. 5 is a sectional view of the object to be processed taken along theline V-V of FIG. 3;

FIG. 6 is a plan view of the object to be processed cut by the laserprocessing method in accordance with the embodiment;

FIG. 7 is a graph showing relationships between the electric fieldintensity and crack spot size in the laser processing method inaccordance with the embodiment;

FIG. 8 is a sectional view of the object to be processed in a first stepof the laser processing method in accordance with the embodiment;

FIG. 9 is a sectional view of the object to be processed in a secondstep of the laser processing method in accordance with the embodiment;

FIG. 10 is a sectional view of the object to be processed in a thirdstep of the laser processing method in accordance with the embodiment;

FIG. 11 is a sectional view of the object to be processed in a fourthstep of the laser processing method in accordance with the embodiment;

FIG. 12 is a view showing a photograph of a cut section in a part of asilicon wafer cut by the laser processing method in accordance with theembodiment;

FIG. 13 is a graph showing relationships between the laser lightwavelength and the internal transmittance of a silicon substrate in thelaser processing method in accordance with the embodiment;

FIG. 14 is a schematic diagram of the laser processing apparatus inaccordance with Example 1;

FIG. 15 is a flowchart for explaining the laser processing method inaccordance with Example 1;

FIG. 16 is a view showing the semiconductor substrate after a step offorming a starting point region for cutting in accordance with Example1;

FIG. 17 is a view for explaining a step of attaching a protective filmin accordance with Example 1;

FIG. 18 is a view for explaining a step of grinding the semiconductorsubstrate in accordance with Example 1;

FIG. 19 is a view for explaining a step of attaching an expansion filmin accordance with Example 1;

FIG. 20 is a view for explaining a step of peeling the protective filmin accordance with Example 1;

FIG. 21 is a view for explaining a step of expanding the expansion filmand picking up semiconductor chips in accordance with Example 1;

FIG. 22 is a view showing chamfers formed at edge parts on the rear faceside of cut surfaces of semiconductor chips after the step of grindingthe semiconductor substrate in accordance with Example 1;

FIG. 23A is a view for explaining a case where a molten processed regionremains in a cut surface of a semiconductor chip after the step ofgrinding the semiconductor substrate in accordance with Example 1, whilea fracture reaches the front face before the step of grinding thesemiconductor substrate;

FIG. 23B is a view for explaining a case where a molten processed regionremains in a cut surface of a semiconductor chip after the step ofgrinding the semiconductor substrate in accordance with Example 1, whilea fracture does not reach the front face before the step of grinding thesemiconductor substrate;

FIG. 24A is a view for explaining a case where a molten processed regiondoes not remain in a cut surface of a semiconductor chip after the stepof grinding the semiconductor substrate in accordance with Example 1,while a fracture reaches the front face before the step of grinding thesemiconductor substrate;

FIG. 24B is a view for explaining a case where a molten processed regiondoes not remain in a cut surface of a semiconductor chip after the stepof grinding the semiconductor substrate in accordance with Example 1,while a fracture does not reach the front face before the step ofgrinding the semiconductor substrate;

FIG. 25A is a view for explaining a case where a molten processed regionremains in an edge part on the rear face side of a cut surface of asemiconductor chip after the step of grinding the semiconductorsubstrate in accordance with Example 1, while a fracture reaches thefront face before the step of grinding the semiconductor substrate;

FIG. 25B is a view for explaining a case where a molten processed regionremains in an edge part on the rear face side of a cut surface of asemiconductor chip after the step of grinding the semiconductorsubstrate in accordance with Example 1, while a fracture does not reachthe front face before the step of grinding the semiconductor substrate;

FIG. 26A is a sectional view of a marginal part of the semiconductorsubstrate before the step of grinding the semiconductor substrate inaccordance with Example 1;

FIG. 26B is a sectional view of the marginal part of the semiconductorsubstrate after the step of grinding the semiconductor substrate inaccordance with Example 1;

FIG. 27 is a plan view of the sapphire substrate in accordance withExample 2;

FIG. 28 is a sectional view for explaining a step of forming a startingpoint region for cutting in accordance with Example 2;

FIG. 29 is a sectional view for explaining a step of forming afunctional device in accordance with Example 2;

FIG. 30 is a sectional view for explaining a step of attaching aprotective film in accordance with Example 2;

FIG. 31 is a sectional view for explaining a step of grinding thesapphire substrate in accordance with Example 2;

FIG. 32 is a sectional view for explaining a step of attaching anexpansion film in accordance with Example 2;

FIG. 33 is a sectional view for explaining a step of irradiating theprotective film with UV rays in accordance with Example 2;

FIG. 34 is a sectional view for explaining a step of peeling theprotective film in accordance with Example 2; and

FIG. 35 is a sectional view for explaining a step of expanding theexpansion film and separating semiconductor chips in accordance withExample 2.

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, a preferred embodiment of the present invention willbe explained in detail with reference to drawings. The substratedividing method in accordance with this method comprises the steps ofirradiating a substrate with laser light while positioning alight-converging point within the substrate, so as to form a modifiedregion due to multiphoton absorption within the substrate, therebyforming a starting point region for cutting; and then grinding thesubstrate such that the substrate attains a predetermined thickness.

First, a laser processing method carried out in the step of forming thestarting point region for cutting, multiphoton absorption in particular,will be explained.

A material becomes optically transparent if its absorption bandgap E_(G)is greater than a photon energy hv. Hence, the condition under whichabsorption occurs in the material is hv>E_(G). However, even whenoptically transparent, the material yields absorption under thecondition of nhv>E_(G) (n=2, 3, 4, . . . ) if the intensity of laserlight is very high. This phenomenon is known as multiphoton absorption.In the case of pulse waves, the intensity of laser light is determinedby the peak power density (W/cm²) of laser light at a light-convergingpoint thereof. The multiphoton absorption occurs, for example, at a peakpower density (W/cm²) of 1×10⁸ (W/cm²) or higher. The peak power densityis determined by (energy per pulse of laser light at thelight-converging point)/(laser light beam spot cross-sectionalarea×pulse width). In the case of a continuous wave, the intensity oflaser light is determined by the electric field strength (W/cm²) oflaser light at the light-converging point.

The principle of laser processing in accordance with the embodimentutilizing such multiphoton absorption will now be explained withreference to FIGS. 1 to 6. FIG. 1 is a plan view of a substrate 1 duringlaser processing; FIG. 2 is a sectional view of the substrate 1 takenalong the line II-II of FIG. 1; FIG. 3 is a plan view of the substrate 1after laser processing; FIG. 4 is a sectional view of the substrate 1taken along the line IV-IV of FIG. 3; FIG. 5 is a sectional view of thesubstrate 1 taken along the line V-V of FIG. 3; and FIG. 6 is a planview of the cut substrate 1.

As shown in FIGS. 1 and 2, the front face 3 of the substrate 1 has adesirable line along which the substrate should be cut 5 for cutting thesubstrate 1. The line along which the substrate should be cut 5 is alinearly extending virtual line (the substrate 1 may also be formed withan actual line acting as the line along which the substrate should becut 5). In the laser processing in accordance with this embodiment, thesubstrate 1 is irradiated with laser light L such that alight-converging point P is positioned within the semiconductorsubstrate 1 under a condition causing multiphoton absorption, so as toform a modified region 7. Here, the light-converging point is a locationwhere the laser light L is converged.

The laser light L is relatively moved along the line along which thesubstrate should be cut 5 (in the direction of arrow A), so as to movethe light-converging point P along the line along which the substrateshould be cut 5. This forms the modified region 7 along the line alongwhich the substrate should be cut 5 only within the substrate 1 as shownin FIGS. 3 to 5, and the modified region 7 forms a starting point regionfor cutting (part to cut) 8. In the laser processing method inaccordance with this embodiment, no modified region 7 is formed uponheating the substrate 1 by causing the substrate 1 to absorb the laserlight L. Instead, the laser light L is transmitted through thesemiconductor substrate 1, so as to generate multiphoton absorptionwithin the semiconductor substrate 1, thereby forming the modifiedregion 7. Hence, the laser light L is hardly absorbed by the front face3 of the semiconductor substrate 1, whereby the front face 3 of thesemiconductor substrate 1 does not melt.

If a start point exists at a location to cut when cutting the substrate1, the substrate 1 fractures from this start point and thus can be cutwith a relatively small force as shown in FIG. 6. This makes it possibleto cut the substrate 1 without generating unnecessary fractures in thefront face 3 of the substrate 1.

There seem to be the following two ways of cutting the substrate fromthe starting point region for cutting acting as a start point. The firstcase is where, after forming the starting point region for cutting, anartificial force is applied to the substrate, so that the substratefractures from the starting point region for cutting acting as a startpoint, whereby the substrate is cut. This is the cutting in the casewhere the substrate has a large thickness, for example. The applicationof an artificial force encompasses application of bending stress andshearing stress along the starting point region for cutting of thesubstrate, and exertion of a temperature difference upon the substrateto generate thermal stress, for example. The other case is where astarting point region for cutting is formed, so that the substrate isnaturally fractured in a cross-sectional direction (thickness direction)of the substrate from the starting point region for cutting acting as astart point, whereby the substrate is cut. This is enabled, for example,by forming the starting point region for cutting by a single row ofmodified regions when the substrate has a small thickness, and by aplurality of rows of modified regions aligned in the thickness directionwhen the substrate has a large thickness. Even in the case of naturalfracturing, fractures do not extend to the front face at a location notformed with the starting point region for cutting in the part to cut,whereby only the part corresponding to the location formed with thestarting point region for cutting can be fractured. Thus, fracturing canbe regulated well. Such a fracturing method with favorablecontrollability is quite effective, since semiconductor substrates suchas silicon wafers have recently been apt to become thinner.

The modified region formed by multiphoton absorption in this embodimentincludes the following cases (1) to (3):

(1) Case where the Modified Region is a Crack Region Including One or aPlurality of Cracks

A substrate (e.g., glass or a piezoelectric material made of LiTaO₃) isirradiated with laser light while a light-converging point is positionedtherewithin under a condition with an electric field intensity of atleast 1×10⁸ (W/cm²) at the light-converging point and a pulse width of 1μs or less. This pulse width is a condition under which a crack regioncan be formed only within the substrate while generating multiphotonabsorption without causing unnecessary damages to the substrate. Thisgenerates a phenomenon of optical damage due to multiphoton absorptionwithin the substrate. This optical damage induces thermal distortionwithin the substrate, thereby forming a crack region therewithin. Theupper limit of electric field intensity is 1×10¹² (W/cm²), for example.The pulse width is preferably 1 ns to 200 ns, for example. The formingof a crack region due to multiphoton absorption is described, forexample, in “Internal Marking of Glass Substrate by Solid-state LaserHarmonics”, Proceedings of 45th Laser Materials Processing Conference(Dec. 1998), pp. 23-28.

The inventors determined relationships between the electric fieldintensity and the magnitude of crack by an experiment. Conditions forthe experiment are as follows:

(A) Substrate: Pyrex (registered trademark) glass (having a thickness of700 μm)

(B) Laser

-   -   Light source: semiconductor laser pumping Nd:YAG laser    -   Wavelength: 1064 nm    -   Laser light spot cross-sectional area: 3.14×10⁻⁸ cm²    -   Oscillation mode: Q-switch pulse    -   Repetition frequency: 100 kHz    -   Pulse width: 30 ns    -   Output: output <1 mJ/pulse    -   Laser light quality: TEM₀₀    -   Polarization characteristic: linear polarization

(C) Light-converging lens

-   -   Transmittance with respect to laser light wavelength: 60%

(D) Moving speed of a mounting table mounting the substrate: 100 mm/sec

Here, the laser light quality being TEM₀₀ indicates that the lightconvergence is so high that light can be converged up to about thewavelength of laser light.

FIG. 7 is a graph showing the results of the above-mentioned experiment.The abscissa indicates peak power density. Since laser light is pulselaser light, its electric field intensity is represented by the peakpower density. The ordinate indicates the size of a crack part (crackspot) formed within the substrate processed by one pulse of laser light.Crack spots gather, so as to form a crack region. The size of a crackspot refers to that of the part of dimensions of the crack spot yieldingthe maximum length. The data indicated by black circles in the graphrefers to a case where the light-converging lens (C) has a magnificationof ×100 and a numerical aperture (NA) of 0.80. On the other hand, thedata indicated by white circles in the graph refers to a case where thelight-converging lens (C) has a magnification of ×50 and a numericalaperture (NA) of 0.55. It is seen that crack spots begin to occur withinthe substrate when the peak power density reaches about 10¹¹ (W/cm²),and become greater as the peak power density increases.

A mechanism by which the substrate is cut upon formation of a crackregion in the laser processing in accordance with this embodiment willnow be explained with reference to FIGS. 8 to 11. As shown in FIG. 8,the substrate 1 is irradiated with laser light L while positioning thelight-converging point P within the substrate 1 under a condition wheremultiphoton absorption occurs, so as to form a crack region 9therewithin along a line along which the substrate should be cut. Thecrack region 9 is a region including one or a plurality of crack spots.The crack region 9 forms a starting point region for cutting. As shownin FIG. 9, the crack further grows while using the crack region 9 as astart point (i.e., using the starting point region for cutting as astart point). As shown in FIG. 10, the crack reaches the front face 3and rear face 21 of the substrate 1. As shown in FIG. 11, the substrate1 breaks, so as to be cut. The crack reaching the front face and rearface of the substrate may grow naturally or grow as a force is appliedto the substrate.

(2) Case where the Modified Region is a Molten Processed Region

A substrate (e.g., a semiconductor material such as silicon) isirradiated with laser light while a light-converging point is positionedtherewithin under a condition with an electric field intensity of atleast 1×10⁸ (W/cm²) at the light-converging point and a pulse width of 1μs or less. As a consequence, the inside of the substrate is locallyheated by multiphoton absorption. This heating forms a molten processedregion within the substrate. The molten processed region refers to aregion once melted and then re-solidified, a region just in a meltedstate, or a region in the process of re-solidifying from its meltedstate, and may also be defined as a phase-changed region or a regionhaving changed its crystal structure. The molten processed region mayalso be regarded as a region in which a certain structure has changedinto another structure in monocrystal, amorphous, and polycrystalstructures. Namely, it refers to a region in which a monocrystalstructure has changed into an amorphous structure, a region in which amonocrystal structure has changed into a polycrystal structure, and aregion in which a monocrystal structure has changed into a structureincluding an amorphous structure and a polycrystal structure, forexample. When the substrate is a silicon monocrystal structure, themolten processed region is an amorphous silicon structure, for example.The upper limit of electric field intensity is 1×10¹² (W/cm²), forexample. The pulse width is preferably 1 ns to 200 ns, for example.

By an experiment, the inventors have verified that a molten processedregion is formed within a silicon wafer. Conditions for the experimentare as follows:

(A) Substrate: silicon wafer (having a thickness of 350 μm and an outerdiameter of 4 inches)

(B) Laser

-   -   Light source: semiconductor laser pumping Nd:YAG laser    -   Wavelength: 1064 nm    -   Laser light spot cross-sectional area: 3.14×10⁻⁸ cm²    -   Oscillation mode: Q-switch pulse    -   Repetition frequency: 100 kHz    -   Pulse width: 30 ns    -   Output: 20 μJ/pulse    -   Laser light quality: TEM₀₀    -   Polarization characteristic: linear polarization

(C) Light-converging lens

-   -   Magnification: ×50    -   N. A.: 0.55    -   Transmittance with respect to laser light wavelength: 60%

(D) Moving speed of a mounting table mounting the substrate: 100 mm/sec

FIG. 12 is a view showing a photograph of a cut section in a part of asilicon wafer cut by laser processing under the above-mentionedconditions. A molten processed region 13 is formed within a siliconwafer 11. The size of the molten processed region 13 formed under theabove-mentioned conditions is about 100 μm in the thickness direction.

The fact that the molten processed region 13 is formed by multiphotonabsorption will now be explained. FIG. 13 is a graph showingrelationships between the wavelength of laser light and thetransmittance within the silicon substrate. Here, respective reflectingcomponents on the front face side and rear face side of the siliconsubstrate are eliminated, whereby only the transmittance therewithin isrepresented. The above-mentioned relationships are shown in the caseswhere the thickness t of the silicon substrate is 50 μm, 100 μm, 200 μm,500 μm, and 1000 μm, respectively.

For example, it is seen that laser light transmits through the siliconsubstrate by at least 80% at 1064 nm, where the wavelength of Nd:YAGlaser is located, when the silicon substrate has a thickness of 500 μmor less. Since the silicon wafer 11 shown in FIG. 12 has a thickness of350 μm, the molten processed region 13 due to multiphoton absorption isformed near the center of the silicon wafer, i.e., at a part separatedfrom the front face by 175 μm. The transmittance in this case is 90% orgreater with reference to a silicon wafer having a thickness of 200 μm,whereby the laser light is absorbed within the silicon wafer 11 onlyslightly and is substantially transmitted therethrough. This means thatthe molten processed region 13 is not formed by laser light absorptionwithin the silicon wafer 11 (i.e., not formed upon usual heating withlaser light), but by multiphoton absorption. The forming of a moltenprocessed region by multiphoton absorption is described, for example, in“Processing Characteristic Evaluation of Silicon by Picosecond PulseLaser”, Preprints of the National Meeting of Japan Welding Society, No.66 (Apr. 2000), pp. 72-73.

Here, a fracture is generated in the cross-sectional direction whileusing a molten processed region as a start point, whereby the siliconwafer is cut when the fracture reaches the front face and rear face ofthe silicon wafer. The fracture reaching the front face and rear face ofthe silicon wafer may grow naturally or grow as a force is applied tothe silicon wafer. The fracture naturally grows from the starting pointregion for cutting to the front face and rear face of the silicon waferin any of the cases where the fracture grows from the molten processedregion in a melted state and where the fracture grows from the moltenprocessed region in the process of re-solidifying from the melted state.In any of these cases, the molten processed region is formed only withinthe silicon wafer. In the cut section after cutting, the moltenprocessed region is formed only therewithin as shown in FIG. 12. When amolten processed region is formed within the substrate, unnecessaryfractures deviating from a line along which the substrate should be cutare hard to occur at the time of fracturing, which makes it easier tocontrol the fracturing.

(3) Case where the Modified Region is a Refractive Index Change Region

A substrate (e.g., glass) is irradiated with laser light while alight-converging point is positioned therewithin under a condition withan electric field intensity of at least 1×10⁸ (W/cm²) at thelight-converging point and a pulse width of 1 ns or less. Whenmultiphoton absorption is generated within the substrate with a veryshort pulse width, the energy caused by multiphoton absorption is nottransformed into thermal energy, so that a permanent structural changesuch as ionic valence change, crystallization, or polarizationorientation is induced within the substrate, whereby a refractive indexchange region is formed. The upper limit of electric field intensity is1×10¹² (W/cm²), for example. The pulse width is preferably 1 ns or less,more preferably 1 ps or less, for example. The forming of a refractiveindex change region by multiphoton absorption is described, for example,in “Formation of Photoinduced Structure within Glass by FemtosecondLaser Irradiation”, Proceedings of 42th Laser Materials ProcessingConference (Nov. 1997), pp. 105-111.

The cases of (1) to (3) are explained as modified regions formed bymultiphoton absorption in the foregoing. When a starting point regionfor cutting is formed as follows in view of the crystal structure of thesubstrate, the cleavage property thereof, and the like, the substratecan be cut with a smaller force and a higher accuracy while using thestarting point region for cutting as a start point.

Namely, in the case of a substrate made of a monocrystal semiconductorhaving a diamond structure such as silicon, the starting point regionfor cutting is preferably formed in a direction along the (111) plane(first cleavage plane) or (110) plane (second cleavage plane). In thecase of a substrate made of a III-V family compound semiconductor havinga zinc ore type structure such as GaAs, the starting point region forcutting is preferably formed in a direction along the (110) plane. Inthe case of a substrate having a hexagonal crystal structure such assapphire (Al₂O₃), a starting point region for cutting is preferablyformed in a direction along the (1120) plane (A plane) or (1100) plane(M plane) while using the (0001) plane (C plane) as a principal plane.

When the substrate is formed with an orientation flat along a directionto be formed with the starting point region for cutting (e.g., in adirection along the (111) plane in the monocrystal silicon substrate) ora direction orthogonal to the direction to be formed with the startingpoint region for cutting, the starting point region for cuttingextending along the direction to be formed with the starting pointregion for cutting can be formed in the substrate in an easy andaccurate manner with reference to the orientation flat.

In the following, the present invention will be explained morespecifically with reference to Examples.

EXAMPLE 1

Example 1 of the substrate dividing method in accordance with thepresent invention will now be explained. Example 1 is directed to a casewhere the substrate 1 is a silicon wafer (having a thickness of 350 μmand an outer diameter of 4 inches) (“substrate 1” will hereinafter bereferred to as “semiconductor substrate 1” in Example 1), whereas thefront face 3 of the semiconductor substrate 1 is formed with a pluralityof functional devices in a device manufacturing process.

First, before explaining a step of forming a starting point region forcutting within the semiconductor substrate 1, a laser processingapparatus employed in the step of forming a starting point region forcutting will be explained with reference to FIG. 14. FIG. 14 is aschematic diagram of the laser processing apparatus 100.

The laser processing apparatus 100 comprises a laser light source 101for generating laser light L; a laser light source controller 102 forcontrolling the laser light source 101 so as to regulate the output,pulse width, etc. of laser light L and the like; a dichroic mirror 103,arranged so as to change the orientation of the optical axis of laserlight L by 90°, having a function of reflecting the laser light L; alight-converging lens 105 for converging the laser light L reflected bythe dichroic mirror 103; a mounting table 107 for mounting asemiconductor substrate 1 irradiated with the laser light L converged bythe light-converging lens 105; an X-axis stage 109 for moving themounting table 107 in the X-axis direction; a Y-axis stage 111 formoving the mounting table 107 in the Y-axis direction orthogonal to theX-axis direction; a Z-axis stage 113 for moving the mounting table 107in the Z-axis direction orthogonal to the X- and Y-axis directions; anda stage controller 115 for controlling the movement of these threestages 109, 111, 113.

The Z-axis direction is a direction orthogonal to the front face 3 ofthe semiconductor substrate 1, and thus becomes the direction of focaldepth of laser light L incident on the semiconductor substrate 1.Therefore, moving the Z-axis stage 113 in the Z-axis direction canposition the light-converging point P of laser light L within thesemiconductor substrate 1. This movement of light-converging point P inX(Y)-axis direction is effected by moving the semiconductor substrate 1in the X(Y)-axis direction by the X(Y)-axis stage 109 (111).

The laser light source 101 is an Nd:YAG laser generating pulse laserlight. Known as other kinds of laser usable as the laser light source101 include Nd:YVO₄ laser, Nd:YLF laser, and titanium sapphire laser.For forming a molten processed region, Nd:YAG laser, Nd:YVO₄ laser, andNd:YLF laser are preferably employed. Though pulse laser light is usedfor processing the semiconductor substrate 1 in Example 1, continuouswave laser light may be used as long as it can cause multiphotonabsorption.

The laser processing apparatus 100 further comprises an observationlight source 117 for generating a visible light beam for irradiating thesemiconductor substrate 1 mounted on the mounting table 107, and avisible light beam splitter 119 disposed on the same optical axis asthat of the dichroic mirror 103 and light-converging lens 105. Thedichroic mirror 103 is disposed between the beam splitter 119 andlight-converging lens 105. The beam splitter 119 has a function ofreflecting about a half of a visual light beam and transmitting theremaining half therethrough, and is arranged so as to change theorientation of the optical axis of the visual light beam by 90°. About ahalf of the visible light beam generated from the observation lightsource 117 is reflected by the beam splitter 119, and thus reflectedvisible light beam is transmitted through the dichroic mirror 103 andlight-converging lens 105, so as to illuminate the front face 3 of thesemiconductor substrate 1 including the line along which the substrateshould be cut 5 and the like.

The laser processing apparatus 100 further comprises an image pickupdevice 121 and an imaging lens 123 which are disposed on the sameoptical axis as that of the beam splitter 119, dichroic mirror 103, andlight-converging lens 105. An example of the image pickup device 121 isa CCD camera. The reflected light of the visual light beam havingilluminated the front face 3 including the line along which thesubstrate should be cut 5 and the like is transmitted through thelight-converging lens 105, dichroic mirror 103, and beam splitter 119and forms an image by way of the imaging lens 123, whereas thus formedimage is captured by the image pickup device 121, so as to yield imagingdata.

The laser processing apparatus 100 further comprises an imaging dataprocessor 125 for inputting the imaging data outputted from the imagepickup device 121, an overall controller 127 for controlling the laserprocessing apparatus 100 as a whole, and a monitor 129. According to theimaging data, the imaging data processor 125 calculates focal point datafor positioning the focal point of the visible light generated from theobservation light source 117 onto the front face 3. According to thefocal point data, the stage controller 115 controls the movement of theZ-axis stage 113, so that the focal point of visible light is positionedon the front face 3. Hence, the imaging data processor 125 functions asan autofocus unit. Also, according to the imaging data, the imaging dataprocessor 125 calculates image data such as an enlarged image of thefront face 3. The image data is sent to the overall controller 127,subjected to various kinds of processing therein, and then sent to themonitor 129. As a consequence, an enlarged image or the like isdisplayed on the monitor 129.

Data from the stage controller 115, image data from the imaging dataprocessor 125, and the like are fed into the overall controller 127.According to these data as well, the overall controller 127 regulatesthe laser light source controller 102, observation light source 117, andstage controller 115, thereby controlling the laser processing apparatus100 as a whole. Thus, the overall controller 127 functions as a computerunit.

With reference to FIGS. 14 and 15, a step of forming a starting pointregion for cutting in the case using the above-mentioned laserprocessing apparatus 100 will be explained. FIG. 15 is a flowchart forexplaining the step of forming a starting point region for cutting.

Light absorption characteristics of the semiconductor substrate 1 aredetermined by a spectrophotometer or the like which is not depicted.According to the results of measurement, a laser light source 101generating laser light L having a wavelength to which the semiconductorsubstrate 1 is transparent or exhibits a low absorption is chosen(S101). Subsequently, the thickness of the semiconductor substrate 1 ismeasured. According to the result of measurement of thickness and therefractive index of the semiconductor substrate 1, the amount ofmovement of the semiconductor substrate 1 in the Z-axis direction isdetermined (S103). This is an amount of movement of the semiconductorsubstrate 1 in the Z-axis direction with reference to thelight-converging point P of laser light L positioned at the front face 3of the semiconductor substrate 1 in order for the light-converging pointP of laser light L to be positioned within the semiconductor substrate1. This amount of movement is fed into the overall controller 127.

The semiconductor substrate 1 is mounted on the mounting table 107 ofthe laser processing apparatus 100. Subsequently, visible light isgenerated from the observation light source 117, so as to illuminate thesemiconductor substrate 1 (S105). The illuminated front face 3 of thesemiconductor substrate 1 including the line along which the substrateshould be cut 5 is captured by the image pickup device 121. The linealong which the substrate should be cut 5 is a desirable virtual linefor cutting the semiconductor substrate 1. Here, in order to obtainsemiconductor chips by dividing the semiconductor substrate 1 into thefunctional devices formed on its front face 3, the line along which thesubstrate should be cut 5 is set like a grid running between thefunctional devices adjacent each other. The imaging data captured by theimaging device 121 is sent to the imaging data processor 125. Accordingto the imaging data, the imaging data processor 125 calculates suchfocal point data that the focal point of visible light from theobservation light source 117 is positioned at the front face 3 (S107).

The focal point data is sent to the stage controller 115. According tothe focal point data, the stage controller 115 moves the Z-axis stage113 in the Z-axis direction (S109). As a consequence, the focal point ofvisible light from the observation light source 117 is positioned at thefront face 3 of the semiconductor substrate 1. According to the imagingdata, the imaging data processor 125 calculates enlarged image data ofthe front face 3 of the semiconductor substrate 1 including the linealong which the substrate should be cut 5. The enlarged image data issent to the monitor 129 by way of the overall controller 127, whereby anenlarged image of the line along which the substrate should be cut 5 andits vicinity is displayed on the monitor 129.

Movement amount data determined in step S103 has been fed into theoverall controller 127 beforehand, and is sent to the stage controller115. According to the movement amount data, the stage controller 115causes the Z-axis stage 113 to move the substrate 1 in the Z-axisdirection to a position where the light-converging point P of laserlight L is positioned within the semiconductor substrate 1 (S111).

Subsequently, laser light L is generated from the laser light source101, so as to irradiate the line along which the substrate should be cut5 in the front face 3 of the semiconductor substrate 1. Then, the X-axisstage 109 and Y-axis stage 111 are moved along the line along which thesubstrate should be cut 5, so as to form a molten processed region alongthe line along which the substrate should be cut 5, thereby forming astarting point region for cutting within the semiconductor substrate 1along the line along which the substrate should be cut 5 (S113).

The foregoing completes the step of forming a starting point region forcutting, thereby forming the starting point region for cutting withinthe semiconductor substrate 1. When the starting point region forcutting is formed within the semiconductor substrate 1, a fracture isgenerated in the thickness direction of the semiconductor substrate 1from the starting point region for cutting acting as a start pointnaturally or with a relatively small force exerted thereon.

In Example 1, the starting point region for cutting is formed at aposition near the front face 3 side within the semiconductor substrate 1in the above-mentioned step of forming a starting point region forcutting, and a fracture is generated in the thickness direction of thesemiconductor substrate 1 from the starting point region for cuttingacting as a start point. FIG. 16 is a view showing the semiconductorsubstrate 1 after the starting point region for cutting is formed. Asshown in FIG. 16, fractures 15 generated from the starting point regionfor cutting acting as a start point are formed like a grid along linesto cut, and reach only the front face 3 of the semiconductor substrate 1but not the rear face 21 thereof. Namely, the fractures 15 generated inthe semiconductor substrate 1 separate a plurality of functional devices19 formed like a matrix on the front face of the semiconductor substrate1 from each other. The cut surfaces of the semiconductor substrate 1 cutby the fractures 15 are in close contact with each other.

Here, “the starting point region for cutting is formed at a positionnear the front face 3 side within the semiconductor substrate 1” meansthat a modified region such as a molten processed region constituting astarting point region for cutting is formed so as to shift from thecenter position in the thickness direction of the semiconductorsubstrate 1 (i.e., half thickness position) toward the front face 3.Namely, it refers to a case where the center position of the width ofthe modified region in the thickness direction of the semiconductorsubstrate 1 is shifted toward the front face 3 from the center positionin the thickness direction of the semiconductor substrate 1, and is notlimited to the case where the whole modified region is located on thefront face 3 side from the center position in the thickness direction ofthe semiconductor substrate 1.

The step of grinding the semiconductor substrate 1 will now be explainedwith reference to FIGS. 17 to 21. FIGS. 17 to 21 are views forexplaining respective steps including the step of grinding thesemiconductor substrate. In Example 1, the semiconductor substrate 1 isthinned from the thickness of 350 μm to a thickness of 50 μm.

As shown in FIG. 17, a protective film 20 is attached to the front face3 of the semiconductor substrate after the starting point region forcutting is formed. The protective film 20 is used for protecting thefunctional devices 19 formed on the front face 3 of the semiconductorsubstrate 1 and holding the semiconductor substrate 1. Subsequently, asshown in FIG. 18, the rear face 21 of the semiconductor substrate 1 issubjected to surface grinding and then chemical etching, whereby thesemiconductor substrate 1 is thinned to the thickness of 50 μm. As aconsequence, i.e., because of the grinding of the rear face 21 of thesemiconductor substrate 1, the rear face 21 reaches the fractures 15generated from the starting point region for cutting acting as a startpoint, whereby the semiconductor substrate 1 is divided intosemiconductor chips 25 having the respective functional devices 19.Examples of the chemical etching include wet etching (HF.HNO₃) andplasma etching (HBr.Cl₂).

Then, as shown in FIG. 19, an expansion film 23 is attached so as tocover the rear faces of all the semiconductor chips 25. Thereafter, asshown in FIG. 20, the protective film 20 attached so as to cover thefunctional devices of all the semiconductor chips 25 are peeled off.Subsequently, as shown in FIG. 21, the expansion film 23 is expanded, sothat the semiconductor chips 25 are separated from each other, and asuction collet 27 picks up the semiconductor chips 25.

As explained in the foregoing, the substrate dividing method inaccordance with Example 1 can grind the rear face 21 of thesemiconductor substrate 1 after forming the functional devices 19 on thefront face 3 of the semiconductor substrate 1 in the devicemanufacturing process. Also, because of the following effectsrespectively exhibited by the step of forming a starting point regionfor cutting and the step of grinding the semiconductor substrate, thesemiconductor chips 25 thinned so as to respond to the smaller size ofsemiconductor devices can be obtained with a favorable yield.

Namely, the step of forming a starting point region for cutting canprevent unnecessary fractures and melting deviated from a desirable linealong which the substrate should be cut for cutting the semiconductorsubstrate 1 from occurring, and thus can keep unnecessary fractures andmelting from occurring in the semiconductor chips 25 obtained bydividing the semiconductor substrate 1.

The step of forming a starting point region for cutting does not meltthe front face 3 of the semiconductor substrate 1 along the line alongwhich the substrate should be cut, and thus can narrow the gap betweenthe functional devices 19 adjacent each other, thereby making itpossible to increase the number of semiconductor chips 25 separated fromone semiconductor substrate 1.

On the other hand, the step of grinding the semiconductor substratesubjects the rear face 21 of the semiconductor substrate 1 to surfacegrinding such that the semiconductor substrate 1 attains a predeterminedthickness after the starting point region for cutting is formed withinthe semiconductor substrate 1. Here, even if the rear face 21 reachesthe fractures 15 generated from the starting point region for cuttingacting as a start point, the cut surfaces of the semiconductor substrate1 cut by the fractures 15 are in close contact with each other, wherebythe semiconductor substrate 1 can be prevented from chipping andcracking because of the surface grinding. Therefore, the semiconductorsubstrate 1 can be made thinner and divided while preventing thechipping and cracking from occurring.

The close contact of the cut surfaces in the semiconductor substrate 1is also effective in preventing the grinding dust caused by the surfacegrinding from entering the fractures 15, and keeping the semiconductorchips 25 obtained by dividing the semiconductor substrate 1 from beingcontaminated with the grinding dust. Similarly, the close contact of thecut surfaces in the semiconductor substrate 1 is effective in reducingthe chip-off of the semiconductor chips 25 caused by the surfacegrinding as compared with the case where the semiconductor chips 25 areseparated from each other. Namely, as the protective film 20, one with alow holding power can be used.

Since the rear face 21 of the semiconductor substrate 1 is subjected tochemical etching, the rear faces of the semiconductor chips 25 obtainedby dividing the semiconductor substrate 1 can be made smoother. Further,since the cut surfaces of the semiconductor substrate 1 caused by thefractures 15 generated from the starting point region for cutting actingas a start point are in close contact with each other, only edge partsof the cut surfaces on the rear face side are selectively etched asshown in FIG. 22, whereby chamfers 29 are formed. Therefore, thetransverse rupture strength of the semiconductor chips 25 obtained bydividing the semiconductor substrate 1 can be improved, and the chippingand cracking in the semiconductor chips 25 can be prevented fromoccurring.

The relationship between the semiconductor chip 25 and the moltenprocessed region 13 after the step of grinding the semiconductorsubstrate includes those shown in FIGS. 23A to 25B. The semiconductorchips 25 shown in these drawings have their respective effects explainedlater, and thus can be used according to various purposes. FIGS. 23A,24A, and 25A show the case where the fracture 15 reaches the front face3 of the semiconductor substrate 1 before the step of grinding thesemiconductor substrate, whereas FIGS. 23B, 24B, and 25B show the casewhere the fracture 15 does not reach the front face 3 of thesemiconductor substrate 1 before the step of grinding the semiconductorsubstrate. Even in the case of FIGS. 23B, 24B, and 25B, the fracture 15reaches the front face 3 of the semiconductor substrate 1 after the stepof grinding the semiconductor substrate.

In the semiconductor chip 25 having the molten processed region 13remaining within the cut surface as shown in FIGS. 23A and 23B, the cutsurface is protected by the molten processed region 13, whereby thetransverse rupture strength of the semiconductor chip 25 improves.

The semiconductor chip 25 in which the molten processed region 13 doesnot remain within the cut surface as shown in FIGS. 24A and 24B iseffective in the case where the molten processed region 13 does notfavorably influence the semiconductor device.

In the semiconductor chip 25 in which the molten processed region 13remains in an edge part on the rear face side of the cut surface asshown in FIGS. 25A and 25B, the edge part is protected by the moltenprocessed region 13, whereby the chipping and cracking in the edge partcan be prevented from occurring as in the case where the edge part ofthe semiconductor chip 25 is chamfered.

The rectilinearity of the cut surface obtained after the step ofgrinding the semiconductor substrate improves more in the case where thefracture 15 does not reach the front face 3 of the semiconductorsubstrate 1 before the step of grinding the semiconductor substrate asshown in FIGS. 23B, 24B, and 25B than in the case where the fracture 15reaches the front face 3 of the semiconductor substrate 1 before thestep of grinding the semiconductor substrate as shown in FIGS. 23A, 24A,and 25A.

Whether the fracture reaches the front face 3 of the semiconductorsubstrate 1 or not depends on not only the depth of the molten processedregion 13 from the front face 3, but also the size of the moltenprocessed region 13. Namely, when the molten processed region 13 is madesmaller, the fracture 15 does not reach the front face 3 of thesemiconductor substrate 1 even if the depth of the molten processedregion 13 from the front face 3 is small. The size of the moltenprocessed region 13 can be controlled by the output of the pulse laserlight in the step of forming a starting point region for cutting, forexample, and becomes greater and smaller as the output of the pulselaser light is higher and lower, respectively.

In view of a predetermined thickness of the semiconductor substrate 1thinned in the step of grinding the semiconductor substrate, it ispreferred that marginal parts (outer peripheral parts) of thesemiconductor substrate 1 be rounded by at least the predeterminedthickness by chamfering beforehand (e.g., before the step of forming astarting point region for cutting). FIGS. 26A and 26B are respectivesectional views of a marginal part of the semiconductor substrate 1before and after the step of grinding the semiconductor substrate inaccordance with Example 1. The thickness of the semiconductor 1 shown inFIG. 26A before the step of grinding the semiconductor substrate is 350μm, whereas the thickness of the semiconductor 1 shown in FIG. 26B afterthe step of grinding the semiconductor substrate is 50 μm. As shown inFIG. 26A, a plurality of (seven here) rounded portions are formed at themarginal part of the semiconductor substrate 1 beforehand by chamferingwith a thickness of 50 μm each, i.e., the marginal part of thesemiconductor substrate 1 is caused to have a wavy form. As aconsequence, the marginal part of the semiconductor substrate 1 afterthe step of grinding the semiconductor substrate 1 attains a staterounded by chamfering as shown in FIG. 26B, whereby the chipping andcracking can be prevented from occurring at the marginal part, andhandling can be made easier because of an improvement in mechanicalstrength.

EXAMPLE 2

Example 2 of the substrate dividing method in accordance with thepresent invention will now be explained with reference to FIGS. 27 to35. Example 2 relates to a case where the substrate 1 is a sapphiresubstrate (having a thickness of 450 μm and an outer diameter of 2inches) which is an insulating substrate (“substrate 1” will hereinafterbe referred to as “sapphire substrate 1” in Example 2), so as to yield asemiconductor chip to become a light-emitting diode. FIGS. 28 to 35 aresectional views of the sapphire substrate 1 taken along the line XX-XXof FIG. 27.

First, as shown in FIG. 28, the sapphire substrate 1 is irradiated withlaser light L while a light-converging point P is positionedtherewithin, so as to form a modified region 7 within the sapphiresubstrate 1. In a later step, a plurality of functional devices 19 areformed like a matrix on the front face 3 of the sapphire substrate 1,and the sapphire substrate 1 is divided into the functional devices 19.Therefore, lines to cut are formed like a grid in conformity to the sizeof each functional device 19 as seen from the front face 3 side,modified regions 7 are formed along the lines to cut, and the modifiedregions 7 are used as starting point regions for cutting.

When the sapphire substrate 1 is irradiated with laser light under acondition with an electric field intensity of at least 1×10⁸ (W/cm²) atthe light-converging point P and a pulse width of 1 μs or less, a crackregion is formed as the modified region 7 (there is also a case where amolten processed region is formed). When the (0001) plane of thesapphire substrate 1 is employed as the front face 3, and a modifiedregion 7 is formed in a direction along the (1120) plane and a directionorthogonal thereto, the substrate can be cut by a smaller force with afavorable accuracy from the starting point region for cutting formed bythe modified region 7 as a start point. The same holds when a modifiedregion 7 is formed in a direction along the (1100) plane and a directionorthogonal thereto.

After the starting point region for cutting is formed by the modifiedregion 7, an n-type gallium nitride compound semiconductor layer(hereinafter referred to as “n-type layer”) 31 is grown as a crystaluntil its thickness becomes 6 μm on the front face 3 of the sapphiresubstrate 1, and a p-type gallium nitride compound semiconductor layer(hereinafter referred to as “p-type layer”) 32 is grown as a crystaluntil its thickness becomes 1 μm on the n-type layer 31. Then, then-type layer 31 and p-type layer 32 are etched to the middle of then-type layer 31 along the modified regions 7 formed like a grid, so asto form a plurality of functional devices 19 made of the n-type layer 31and p-type layer 32 into a matrix.

After the n-type layer 31 and p-type layer 32 are formed on the frontface 3 of the sapphire substrate 1, the sapphire substrate 1 may beirradiated with laser light L while the light-converging point P ispositioned therewithin, so as to form the modified regions 7 within thesapphire substrate 1. The sapphire substrate 1 may be irradiated withthe laser light L from the front face 3 side or rear face 21 side. Evenwhen the laser light L is irradiated from the front face 3 side afterthe n-type layer 31 and p-type layer 32 are formed, the n-type layer 31and p-type layer 32 can be prevented from melting, since the laser lightL is transmitted through the sapphire substrate 1, n-type layer 31, andp-type layer 32.

After the functional devices 19 made of the n-type layer 31 and p-typelayer 32 are formed, a protective film 20 is attached to the front face3 side of the sapphire substrate 1. The protective film 20 is used forprotecting the functional devices 19 formed on the front face 3 of thesemiconductor substrate 1 and holding the sapphire substrate 1.Subsequently, as shown in FIG. 31, the rear face 21 of the sapphiresubstrate 1 is subjected to surface grinding, so that the sapphiresubstrate 1 is thinned to the thickness of 150 μm. The grinding of therear face 21 of the sapphire substrate 1 generates a fracture 15 from astarting point region for cutting formed by the modified region 7 as astart point. This fracture 15 reaches the front face 3 and rear face 21of the sapphire substrate 1, whereby the sapphire substrate 1 is dividedinto semiconductor chips 35 each having the functional device 19constituted by the n-type layer 31 and p-type layer 32.

Next, an expandable expansion film 23 is attached so as to cover therear faces of all the semiconductor chips 25 as shown in FIG. 32, andthen the protective film 20 is irradiated with UV rays as shown in FIG.33, so as to cure a UV curable resin which is an adhesive layer of theprotective film 20. Thereafter, the protective film 20 is peeled off asshown in FIG. 34. Subsequently, as shown in FIG. 35, the expansion film23 is expanded outward, so as to separate the semiconductor chips 25from each other, and the semiconductor chips 25 are picked up by asuction collet or the like. Thereafter, electrodes are attached to then-type layer 31 and p-type layer 32 of the semiconductor chip 25, so asto make a light-emitting diode.

In the step of forming a starting point region for cutting in thesubstrate dividing method in accordance with Example 2, as explained inthe foregoing, the sapphire substrate 1 is irradiated with the laserlight L while the light-converging point P is positioned therewithin, soas to form a modified region 7 by generating a phenomenon of multiphotonabsorption, whereby the modified region 7 can form a starting pointregion for cutting within the sapphire substrate 1 along a desirableline along which the substrate should be cut for cutting the sapphiresubstrate 1. When a starting point region for cutting is formed withinthe sapphire substrate 1, a fracture 15 is generated in the thicknessdirection of the sapphire substrate 1 from the starting point region forcutting acting as a start point naturally or with a relatively smallforce exerted thereon.

In the step of grinding the sapphire substrate 1, the sapphire substrate1 is ground so as to attain a predetermined thickness after a startingpoint region for cutting is formed within the sapphire substrate 1.Here, even when the ground surface reaches the fracture 15 generatedfrom the starting point region for cutting acting as a start point, thecut surfaces of the sapphire substrate 1 cut by the fracture 15 are inclose contact with each other, whereby the sapphire substrate 1 can beprevented from chipping and cracking upon grinding.

Therefore, the sapphire substrate 1 can be thinned and divided whilepreventing the chipping and cracking from occurring, wherebysemiconductor chips 25 with the thinned sapphire substrate 1 can beobtained with a favorable yield.

Effects similar to those mentioned above are also obtained when dividinga substrate using an AlN substrate or GaAs substrate instead of thesapphire substrate 1.

INDUSTRIAL APPLICABILITY

As explained in the foregoing, the present invention can thin and dividethe substrate while preventing the chipping and cracking from occurring.

The invention claimed is:
 1. A substrate comprising: a plurality ofmodified regions each constituting a start point region for use incutting the substrate, each modified region being formed within thesubstrate by laser light irradiation along one of a plurality of cuttinglines arranged in a planar grid with respect to the substrate, whereineach of the modified regions is shifted from a center position in athickness direction of the substrate toward one face of the substrate,and fractures generated from the start point which only reach the oneface of the substrate but not the other face of the substrate, whereincut surfaces of the substrate cut by the fractures being in closecontact with each other rather than being separate from each other. 2.The substrate according to claim 1, wherein the substrate comprises asemiconductor substrate.
 3. The substrate according to claim 1, whereinthe substrate comprises a sapphire substrate or an insulated substrateof AlN(aluminum nitride).